Oswalt (7-8) struck out nine and allowed one run and six hits before walking off the field with a trainer after throwing his warmup pitches for the seventh inning. Manager Cecil Cooper said Oswalt strained his left hip abductor in the sixth. Oswalt had grounded out in the bottom of the inning, but catcher Brad Ausmus said Oswalt hurt himself on the last pitch he threw, a curveball.
Cooper said Oswalt was "day-to-day." He is due to start again in Atlanta on Saturday.
"We'll have to wait and see how he is," Cooper said. "Right now, I'll say he's fine and he'll take his next start. We'll just have to wait and see."
Oswalt said he never felt an injury like this before.
"I hope it ain't too bad," said Oswalt, who threw 79 pitches. "I think I came out before it got too bad. I just hope it's nothing but a hip pointer where your ribs hit your hip. We will just see how it feels tomorrow."
James Loney's sacrifice fly in the first inning drove in the Dodgers' only run. Los Angeles has scored two runs in their last three games and one run or fewer in five of its last seven games. The Dodgers didn't get a hit against the Los Angeles Angels on Saturday and managed to win 1-0.
"We're struggling right now," Dodgers manager Joe Torre said. "We really just haven't been able to put any hits together. It's frustrating."
Oswalt had lost four of his last six starts but looked like his dominant self after a shaky first. He gave up a one-out double to Andre Ethier and plunked Russell Martin and Jeff Kent with pitches before Loney's sacrifice fly.
Berkman hit a solo homer to left in the bottom of the first, his 22nd of the season and his fifth batting right-handed. It was the first homer allowed by Stults in three starts this season.
"I left a lot of pitches up," Stults said. "My changeup was in the zone. That's what hurt me tonight."
Oswalt retired the next seven batters before Kent led off the fourth with a single. Miguel Tejada made a sharp backhanded stop on Loney's grounder to start a double play and Oswalt struck out Delwyn Young to end the inning.
Though he's had mixed results lately, Oswalt had pitched at least seven innings in four of his previous six starts.
"I feel better every start," Oswalt said. "The last month, I think I have been down in the zone more and that has allowed me to get more ground balls and get some punch outs to get some guys looking."
Stults set down eight straight before walking Berkman with one out in the fourth. Carlos Lee then hit a blooper to center that skipped away from Jason Repko. Berkman slid into third and Lee took second on what was ruled a double.
Repko was recalled from Triple-A on Monday to replace regular center fielder Juan Pierre, who was placed on the disabled list with a torn knee ligament. Repko went 0-for-5 with four strikeouts.
Torre said Repko was nervous in his first start of the season.
"He was out front and really jumping badly," Torre said.
Stults intentionally walked Tejada to load the bases, but Mark Loretta singled to left to put Houston up 2-1. Loretta had the game-winning hit in Sunday's 3-2 win over Boston and had a three-run homer in Saturday's 11-10 victory. One out later, Ausmus singled to center for a 4-1 lead.
The Dodgers put two runners aboard in the fifth and sixth, but Oswalt escaped both times. He struck out two batters in the fifth and then got Ethier to fly out to end the inning. In the sixth, Oswalt fanned Young and Blake DeWitt to stall the threat.
"That fifth and sixth inning," Cooper said, "he was as good as he's ever been."
Oswalt grounded out to reliever Ramon Troncoso to end the sixth and seemed fine as he got ready to pitch the seventh. Backup Humberto Quintero caught Oswalt while Ausmus was in the dugout putting on his gear.
But Oswalt called Quintero to the mound and, after a brief conversation, Quintero signaled to the dugout. Cooper and assistant trainer Rex Jones came out to check on Oswalt and he walked off the field with Jones.
Chris Sampson came in to relieve and avoided trouble until Loney and Young reached on two-out singles in the eighth. Cooper brought in left-hander Wesley Wright and he got pinch-hitter Andy LaRoche to fly out.
Astros starters have allowed three earned runs or fewer in 12 of the last 14 games. ... The Dodgers had played in five consecutive shutouts before Monday's game. ... The Dodgers' bullpen has not allowed a run in 15 2/3 innings.